1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, and the like for forming an image on a recording material making use of an electrophotographic type process.
2. Description of the Related Art
At present, there are widely used electrophotographic image forming apparatuses having process cartridges, in which photosensitive drums, development units, and the like are arranged integrally with each other and which are detachably attachable thereon in order to execute maintenance and to replenish developers (hereinafter, referred to as “toner”) easily. Since this type of the process cartridge must contain the amounts of toner corresponding to the durable life of the photosensitive drums determined by the deterioration thereof, there is a tendency for the sizes of the process cartridges to increase in their entirety.
Further, the amount of consumption of toner is greatly different depending on the density of an image created by a user. Accordingly, the process cartridge must be replaced when the photosensitive drum is deteriorated to a certain degree and when the toner is consumed in a predetermined amount. Actually, however, since the process cartridge is replaced early regardless of the fact that the photosensitive drum can be still used or toner still remains, there is a tendency that the running cost increases.
To cope with the above problem, recently, an image forming apparatus has been in practical use in which process cartridges are separated from toner cartridges for supplying toner to development units in the process cartridges and the respective cartridges can be independently attachable.
For example, as shown in FIG. 15, four process cartridges 200 for forming yellow, magenta, cyan, and black images and four toner cartridges 201 for supplying toner to the process cartridges 200 are mounted on an apparatus main body 202. Then, the toner images formed on photosensitive drums 200a in the process cartridges 200 are primarily transferred onto an intermediate transfer belt 203 sequentially. Further, the primarily transferred images are transferred onto a transfer material transported from a sheet cassette 204 by a transport roller 205 and rollers 214 in a secondary transfer section 206. After the transfer material is transported to a fixing unit 207 and the toner images are heated and fixed, it is discharged by rollers 216 to a discharge section 208 located to an upper portion of the apparatus (refer to Japanese Patent Application Laid-Open Publication No. 2002-148897).
When the toner cartridge is separated from the process cartridge, the respective cartridges can be replaced as they are consumed according to the durability thereof, thereby reducing the running cost of the cartridges.
Further, when a user, who conventionally uses a monochrome image forming apparatus, replaces it with a color image forming apparatus, the user does not always form a color image, and a monochrome image forming frequency may be higher than a color image forming frequency.
Accordingly, an image forming apparatus has been in practical use in which the frequency of replacing a black toner cartridge is reduced by setting the capacity of the black toner cartridge larger than that of the toner cartridges other than the black toner cartridge (refer to Japanese Patent Application Laid-Open Publication No. 2001-265088).
However, when the capacity of a particular toner cartridge is increased, a portion of the toner cartridge protrudes. When the spaces of the other toner cartridges are also increased to cover the protruded portion of the specific toner cartridge, a problem arises in that the size of an apparatus main body is increased.
An object of the present invention, which was made in view of the above problems, is to provide an image forming apparatus which has toner cartridges and process cartridges separately mounted thereon and can reduce the frequency of replacement of the toner cartridge used in a particular color image forming station without adversely affecting the size of an apparatus main body.